$data_block_size gives the smallest unit of disk space that can be
allocated at a time expressed in units of 512-byte sectors. People
primarily interested in thin provisioning may want to use a value such
as 1024 (512KB). People doing lots of snapshotting may want a smaller value
such as 128 (64KB). If you are not zeroing newly-allocated data,
a larger $data_block_size in the region of 256000 (128MB) is suggested.
$data_block_size must be the same for the lifetime of the
metadata device.

$low_water_mark is expressed in blocks of size $data_block_size. If
free space on the data device drops below this level then a dm event
will be triggered which a userspace daemon should catch allowing it to
extend the pool device. Only one such event will be sent.
Resuming a device with a new table itself triggers an event so the
userspace daemon can use this to detect a situation where a new table
already exceeds the threshold.
とのことなので、最初に設定したThin-pool 100GBのうち、容量が32768 * (512*512)Byte = 8MBytes を切ると、拡張がされるように思います(動作は未確認)
* このあたりは決め打ちで変更はできないようですね